The invention relates generally to high temperature polymers and in particular to phthalonitrile polymers and their cures. More specifically, the present invention relates to di-phthalonitrile polymers cured with the curing agents described herein, to methods of curing di-phthalonitrile monomers to produce phthalonitrile resins with high temperature properties, and to the phthalonitrile prepolymers produced by the methods herein. The curing agents are selected from the group consisting essentially of (a) an acid and an amine, (b) salts of an acid and an amine, and (c). and mixtures of (a) and (b).
It is known that di-phthalonitrile monomers polymerize to produce thermally stable phthalonitrile resins. There is an extensive range of applications for such phthalonitrile resins including as matrix materials for lightweight high temperature resistant carbon fibre composites for use in aircraft engine nacelles, as binding filler suitable for use in clutch or brake linings, and in hot molds for casting.
Interest in fiber-reinforced composites for advanced aerospace applications has led to the search for high-temperature polymers that are easily processed and exhibit high thermal and oxidative stability. Epoxies and polyimides are now being used but each has its disadvantages. Conventional epoxy-based composites and adhesives have a 200.degree. C. maximum service limit and polyimide resins used in composites matrices have a 300.degree. C. maximum service limit. Advanced design concepts, especially in the aerospace industry, demand even higher temperature requirements for polymeric materials.
A major problem of the polyimide system is the inability to process void- and blister-free components in high yield because of the evolution of volatile components formed during the polymerization. Other problems associated with both polyimides and epoxies include their brittleness, water absorptivity and engineering reliability.
Phthalonitrile resins are proving to be superior in physical and chemical properties to epoxies, polyimides and other plastics as matrices for fiber-reinforced composites and in other applications. A major advantage of phthalonitrile resins, compared to other plastics, is their ability to withstand temperatures in excess of 300.degree. C. for extended periods without permanent damage to the coatings, plastics or composites made therefrom. Such resins usually contain a substantial proportion of aromatic structures, but cured polymers composed solely of aromatic rings tend to be brittle and intractable. A resin having flexible linkages between the aromatic rings minimizes or greatly reduces brittleness and intractability. Polyphthalonitrile resins with diether linkages are materials which meet these goals. Examples of these polyphthalonitriles and other polyphthalonitriles are disclosed in the prior art.
One known method of polymerising di-phthalonitrile monomers comprises curing the monomers with metals and metal salts which act as reducing agents to promote the polymerization reaction. However, the phthalonitrile resins produced by this method are not as thermally stable as is required for many applications and there are processing problems associated with the method which are difficult to overcome. For example, a large quantity of metals or metal salts are required for complete reaction.
Another known method of polymerizing di-phthalonitrile monomers comprises curing the monomers at temperatures greater than 300.degree. C. for in excess of several days. The curing time is unacceptably long from the commercial viewpoint.
A known improvement to accelerate the curing time, disclosed in U.S. Pat. Nos. 4,408,035 and 4,410,676 assigned to the United States of America as represented by the Secretary of the Navy, comprises adding a curing additive in the form of small amounts of an active hydrogen source, such as primary amines and phenols, to di-phthalonitrile monomers. For example, U.S. Pat. No. 4,408,035 discloses a method comprising curing a mixture of di-phthalonitrile monomers and a nucleophlic aromatic amine in molar ratio of monomer to amine of 40:1 at a temperature in excess of 200.degree. C. for 24 hours and at 315.degree. C. for a further 24 hours. The resultant phthalonitrile resin, the monomer of which had melting point of 232.degree.-234.degree. C., had good thermal stability and a relatively high glass transition temperature exceeding 200.degree. C. The aromatic diamines covered by U.S. Pat. No. 4,408,035 are somewhat volatile at the required processing temperature and can cause void problems when used in an amount greater than 5% by weight. It is advantageous for a resin not to produce gaseous products on curing.